jekyll: force ruby platform when updating dependencies

Fixes #202506 where an updated jekyll couldn't be used because
the platform-specific and ruby version of the nokogiri dependency
got confused. Inspiration for this fix from
https://github.com/nix-community/bundix/issues/88

pkgs/applications/misc/jekyll/update.sh

···
 for directory in "basic" "full"; do
   pushd "$BASEDIR/$directory"
   rm -f Gemfile.lock gemset.nix
-  bundix --magic
+  BUNDLE_FORCE_RUBY_PLATFORM=true bundix --magic
   rm -rf .bundle vendor
   popd
 done